I need to watch out for this (not using cards) myself.
I tend to use one card only (Amex) unless they won't take it, then my UA mileage card.
The others go unused and have high enough credit limits, but I should probably ping them all once in a while to keep things active. Some of them I got on promo's and used for zero cost balance transfers or purchases (mostly this) and zero interest over xxx time. Then after I paid them off early, got the limits raised, I stopped using them.
P.S. It does suck that I always have a high Amex revolving credit amount showing on the credit agencies records because of the stupid way their billing cycle works with their reporting cycle. Considering there's "no limit" on Amex*, it's kinda stupid.
* It's really based on your credit worthiness and spending habits. FWIW, I charged $25K for some medical bills on my Amex cause I could. Points cha-ching.
FWIW, I pay for one of the monitoring services on one of the big three and pull my new reports/scores for all three nearly every month or two when I think of it. I get weekly e-mail alerts that all is AOK. I can pull the one I pay for as often as i wish, but the free reports/scores for the other two can only be every 30+ days.